There are many applications in which certain physical properties of liquids are measured. Three of the physical properties are viscosity, corrosivity, and conductivity. Viscosity refers to the degree to which a fluid resists flow. For example, at room temperature molasses is far more viscous than water. Corrosivity refers to the degree to which a material reacts chemically with its environment. For example, a piece of iron rusts when exposed to the air. When a material corrodes, it generally gains or loses mass. Conductivity is the degree with which a material conducts electricity. Copper is more conductive than battery acid. Battery acid is more conductive than wood.
Bulk acoustic wave (SAW) sensors are electro acoustic devices that can be used to measure physical or chemical properties. A BAW sensor has a piezoelectric substrate and electrodes. A piezoelectric material is a material that expands or contracts in response to an electric field. Alternatively, squeezing or releasing a piezoelectric material can produce an electric field. Quartz, lithium niobate, lithium tantalite, and lithium iodate are four of the many piezoelectric materials known to those practiced in the arts of acoustic wave devices or piezoelectric materials.
Changing the electrode voltages on a BAW sensor can cause the substrate to expand or contract. Similarly, squeezing and releasing the substrate can cause a voltage difference to appear on the electrodes. Introducing an oscillating electric drive signal across the electrodes can cause an acoustic wave to develop in the substrate. In other words, the substrate vibrates. If the drive signal is suddenly removed, vibration continues. The continuing substrate vibrations cause an output signal to appear on the electrodes. The output signal has the same frequency content as the substrate vibrations and has an amplitude that is proportional to the amplitude of substrate vibrations.
An acoustic wave device also has at least one fundamental frequency. A fundamental frequency is the devices natural frequency. For example, a glass holding water has a fundamental frequency. Tapping the glass produces a sound corresponding to the glass' fundamental frequency. Changing the amount of water in the glass changes the fundamental frequency. Similarly, thickening, thinning, or similarly changing the mass of a BAW sensor can change the BAW sensor's fundamental frequencies.
Corrosion can change the thickness or mass of a BAW sensor. Oftentimes, an electrode includes a material that can corrode. For example, a measurement of the corrosivity of engine oil can be desired. A material that engine oil corrodes can be layered onto an electrode and then the electrode exposed to engine oil. The fundamental frequency of the BAW sensor then changes based on the corrosivity of the engine oil. As such, the corrosivity of a liquid can be measured because it corresponds to the change in a BAW sensor's fundamental frequency.
The viscosity of a liquid, such as the engine oil discussed above, can also be measured by a BAW sensor. The more viscous a liquid is, the more it resists flow. When exposed to a liquid, the amplitude of the BAW sensor vibrations is decreased because all fluids are viscous. The more viscous the liquid is, the smaller the BAW sensor vibrations are. Smaller amplitude BAW vibrations lead to smaller amplitude output signals at the electrodes. As such, the viscosity of a liquid can be measured because it corresponds to the amplitude of the electrode signal.
Liquids also have an electrical conductivity. Electrical conductivity can be measured by placing two electrodes into the liquid, causing a voltage difference between the electrodes, and then measuring the electric current passing from one electrode, through the liquid, and then through the other electrode. The liquid's conductivity can be found as a function of the number of ions per unit volume, electrode spacing, and electrode shape. Those practiced in the art of measuring physical properties know of this and other methods of using electrodes to measure conductivity.
Current technology supplies BAW sensors for measuring corrosivity and viscosity. AT and BT cut quartz crystals are used as piezoelectric substrates. Electrode materials are selected based on their chemical reactivity with a specified liquid. For example, engine oil corrosivity is interesting because the oil is corroding an engine. As such, an electrode material that corrodes similarly to engine components is chosen.
Current technology does not, however, supply an integrated sensor that for measures more than corrosivity and viscosity. In particular, it doesn't supply a sensor that can measure more physical propertied without incurring additional processing steps to construct the sensor. Aspects of the embodiments directly address the shortcoming of current technology by producing an additional structure on an integrated BAW sensor without requiring additional processing steps.